1. Technical Field
The present invention relates to a technology of performing mass spectrometry analysis on ions by using an ion trap, and in particular, to an ion trap analyzer optimized by an auxiliary excitation electric field.
2. Related Art
Since 1953 when Paul invented the three-dimensional quadrupole ion trap technology, as an important part of the mass spectrometry technology, ion traps together with related mass spectrometry technologies are widely used in qualitative and quantitative testing of trace materials and material structure information testing based on fragment dissociation spectra, and are used as ion flow modulation apparatuses of other high-definition pulse ion mass analyzers because the ion traps can keep a large amount of ions under test trapped therein for a long time and eject the ions in a short time to produce a concentration effect. In the history of ion trap apparatuses, the dipole resonance auxiliary excitation mode, as the most important invention, plays a key role in improving the mass resolution performance of an ion trap mass analyzer. In this method, a dipole electric field component is overlaid in an original trapping electric field of the ion trap to improve the orientation of ion ejection, and by means of resonance between an overall motion frequency, namely, a secular motion frequency, of the inherent motion frequency of ions and a frequency of the excitation electric field, the motion range of target ions rises rapidly in a short time during a mass-unstable scanning process, thereby reducing the ejection delay and random collision, which comes along with the ejection delay, between target ions and neutral molecules. Compared with the previous boundary ejection mode which only uses the ion stability condition in a radio frequency (RF) trapping electric field, the dipole resonance auxiliary excitation mode significantly improves the ion ejection efficiency and mass resolution capability. This method has become an indispensable basic technology for commercial analytical instruments of ion trap types.
The dipole resonant excitation mode has been officially applied to commercial instruments since late 1980s. As shown in FIG. 1a, in a 1988 US patent, Syka et al. from Finnigan proposed a three-dimensional rotating ion trap that includes a ring electrode 101 and a pair of end cap electrodes 102 and 103, where an RF voltage V104 can be applied on the ring electrode 101 to generate a quadrupole field, so as to trap ions in two dimensions, namely, a radial direction R and an axial direction, and a dipole alternating voltage V105 is applied between two end caps to excite ions and eject ions selectively, thereby implementing mass scanning. The voltage can also be used as a means to excite the motion range of ions in an applying direction of the alternating voltage, namely, a direction Z, so that the ions collide with other neutral molecules in the ion trap and are broken into fragmented ions. Using the dipole excitation mode to expand an analytical mass-to-charge ratio range of the ion trap has been proposed before. In the dipole excitation mode, a beta value required during ion ejection, that is, a ratio of a double of a secular motion frequency to a frequency of a trapping RF voltage, can be less than 1. Therefore, for ions that are identical in mass-to-charge ratio, the q parameter of an ion ejected in the dipole excitation mode is smaller. In a voltage scanning mode, a smaller q parameter corresponds to a lower ion trapping voltage. Therefore, with the same RF amplitude scanning parameter, a larger mass-to-charge ratio scanning range can be obtained.
People also proposed a two-dimensional linear ion trap to improve the storage capacity of a three-dimensional ion trap. Such an ion trap structure still uses an RF voltage as a trapping voltage. As shown in FIG. 1b ,the ion trap has two pairs of main electrodes 11 and 12 in the direction X and the direction Y. An RF power supply 14 applies high-frequency driving voltages 14.1 and 14.2 that are inverted to each other on the two pairs of main electrodes 11 and 12, so as to form a radial trapping electric field. Ions are generally introduced from one end along the Z axis, and trapped by the electric field in a linear area between the two pairs of electrodes in the X axis and the Y axis. Axial trapping of ions can be implemented relying on an end electrode structure that applies a high potential or by segmenting the main electrodes into multiple sectors and applying a DC trapping bias voltage between sectors. A dipole resonant excitation mode of the two-dimensional linear ion trap is generally implemented by overlaying a dipole excitation voltage in the direction X of the ion trap, where a generator power supply 15 of the voltage is usually overlaid on a main electrode 11.1 on one side of the direction X, while a dipole excitation voltage that is inverted to the voltage on the main electrode 11.1 is overlaid on a main electrode 11.2 on the other side. In this manner, ions can be resonantly excited selectively according to their mass, ejected from a slit 13 between the electrodes in the direction X, and detected by an ion detector installed on the electrode side in the direction X; therefore, mass scanning is implemented.
The resonant dipole excitation mode not only applies to the quadrupole RF ion trap, but also applies to a quadrupole ion trap that uses a static electric field to trap ions, such as the Penning ion trap that traps ions by using a quadrupole static electric field and a static magnetic field jointly, and the currently commercialized Orbitrap that traps ions by using a quadrupole logarithmic field. These different ion traps have a common feature that in an ion excitation or ejection direction X, a function of a trapping potential component applied on ions is V(x)=Ax2; in other words, the field in this direction is a quadratic field, or called a harmonic trap function for short. The secular motion frequency of ions is independent from the resonance amplitude in this direction. Therefore, by applying an excitation alternating electric field whose frequency is the same as a secular frequency of a specific ion trap, a motion-range resonant excitation process of ions can be enabled.
In an ion ejection process of various quadrupole ion traps, a fringing field near an ejection hole has negative influence on simultaneity of ion ejection. Generally, such influence can be indicated by a negative high-order field. That is, when the series of a harmonic function of a pseudo potential of a trap space is expressed as an expansion ΣAnRe(x+yi)n, if the value of n is large (for example, n>5), An will be a negative value due to the said hole, where x is the ion ejection direction, and y is a direction orthogonal to the ejection direction. In the expansion, the term A2 is a quadrupole field component, and the term An is a 2n-pole field component. For an ideal quadrupole ion trap, the expansion of the harmonic function in the ejection direction only includes the term A2, so the ion trapping potential field V(x) in this direction is essentially a quadratic electric field V(x)=A2x2. The ejection outlet can be regarded as a structural deficiency of the RF trapping electrode in the ion ejection direction. In the ion ejection direction, the ion motion is affected by the negative high-order field, which damages the ejection simultaneity of ions having the same mass-to-charge ratio. Such damage is mainly caused by the fact that when the vibration amplitude of ions increases, the restoring force sensed by on the ions is smaller than the force of the simple harmonic potential trap due to the existence of the high-order negative field, and consequently, the resonance frequency of the ions has a red shift, and the resonance of ion motions is detuned.
For many years, people enhance the working performance of the ion trap mainly by improving the field pattern of the trapping electric field. The most direct methods to change the field pattern of the trapping electric field are to modify a boundary structure of a confining electrode of an ion trap. In these methods, the confining electrode in the ejection direction relatively protrudes at the ion ejection outlet, and examples are the solution proposed by Kawato in U.S. Pat. No. 6,087,658, and the method of stretching spacing between confining electrodes in the ejection direction relative to the boundary condition of the ideal quadrupole field.
The trapping electric field may also be improved by dividing the original confining electrode into multiple discrete electrode parts and applying trapping voltages of different amplitudes on these electrode parts. For a three-dimensional ion trap, the inventor of the U.S. Pat. No. 5,468,958 designs a structure having multiple ring electrodes. As shown in FIG. 2a, RF trapping voltages of different proportions are applied on multiple ring electrodes, the proportions of the RF voltages are adjusted by a voltage-dividing capacitor network, and the field pattern can be optimized according to requirements during the experiment. Similarly, for the linear ion trap, Ding Chuanfan designed a linear ion trap enclosed by printed circuit boards in Chinese Patent No. CN1585081. As shown in FIG. 2b, the structure includes multiple discrete adjustable electrode strip patterns, and bounding RF voltages and bounding DC voltages among these electrode patterns are adjusted by a voltage-dividing capacitor-resistor network. As pointed out by Li Gangqiang et al. in the U.S. Pat. No. 7,755,040, a similar method can also be used to construct a static ion trap with an axial quadratic field shown in FIG. 2c. 
In addition, the trapping electric field may also be adjusted by adding a correction electrode. For example, in U.S. Pat. No. 7,279,681, it is proposed to insert a correction electrode in an end cap electrode, and by adjusting the voltage amplitude on the correction electrode, the field pattern in a small area near the ejection hole is optimized. Similarly, in the U.S. Pat. No. 6,608,303, it is proposed to solve the defect of the electric field near the ejection hole by changing the RF voltage phase of the correction electrode added at the ejection outlet.
However, all of the above electric field correction technologies rely on the fact that the voltage can be adjusted by a precisely controlled high-voltage trapping power supply. Such high-voltage power supply may be one commonly called RF resonant power supply, or a high-frequency switch power supply used by a digital ion trap, or may further be a DC power supply in the case of a static ion trap. In any case, the added high-voltage power supply increases the complexity of an instrument; especially, when these high-voltage power supplies are expected to be adjusted discretely, the circuits thereof are even more complex.